During the whole flight, the message "POS ITERV WAS CHGD" appeared frequently on the ACARS page of the MCDU (Multipurpose Control and Display Unit). The aircraft is a Boeing 737. I cannot find any reference in the FCOM (Flight Crew Operating Manual).

What does this message mean and why does it appear?


  • 1
    $\begingroup$ please check netiquette $\endgroup$
    – user8054
    Nov 6, 2019 at 12:27
  • $\begingroup$ Hello gordongly, welcome to Aviation.stackexchange. In the future, please don't use ALL CAPITALS in the title and body of your question, it comes accross as shouting. $\endgroup$
    – DeltaLima
    Nov 6, 2019 at 12:51
  • $\begingroup$ Where did you fly? Do you have ADS-C or CPDLC active? $\endgroup$
    – DeltaLima
    Nov 6, 2019 at 13:03
  • $\begingroup$ China, i think not $\endgroup$
    – gordonlgy
    Nov 6, 2019 at 13:17
  • 2
    $\begingroup$ Assuming it's the real aircraft (doesn't look like a sim), can't your airline ask Boeing through some channel? I always find it odd for RL pilots to ask aircraft-specific questions on the internet $\endgroup$
    – DeepSpace
    Nov 6, 2019 at 15:40

1 Answer 1


Your aircraft is reporting its position periodically over the Aircraft Communications Addressing and Reporting System (ACARS) to the airline operations centre (AOC).

Since there are costs related to sending these position messages, the interval in which these message are sent drives operating costs for the airline.

This patent from China Southern Airlines (original Chinese version here) describes the behaviour you have observed on the MCDU. Apparently it allows the AOC to change the position reporting interval over ACARS remotely. If the interval is changed, a "POS ITERV WAS CHGD" text is displayed on the MCDU scratch pad.

The relevant text from the patent (including translation & typesetting errors):


[0087] PTIC message display unit, see FIG. 6, the module receives PTIC PTIC uplink triggering instruction, parsing PTIC variable, position determined by the adjusted multicast time interval T3, if the variable is valid PTIC, is given in M⑶U unit tip text message to inform the crew of the current position of the broadcast time interval has been changed, the present invention uses "P〇S ITERV wAS CHGD" text gives the unit prompts; the message is always displayed in MCUD the scratch area (Scratchpad) until a new the message will disappear after replace the unit or manually pressing CLR keyboard. ...

  • $\begingroup$ An airline patenting aircraft software. Interesting... $\endgroup$
    – El Ectric
    Nov 6, 2019 at 22:41

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